Many types of sensors are used for tank level detection. For tank level monitoring, magnetostrictive probes are overwhelmingly used to detect various parameters inside of a tank. Notwithstanding this complex task, is the application of precisely determining the fuel and water levels based on the signals resulting from the magnetic interaction between circumferential fields from an applied current in a ferromagnetic wire to those of permanent magnetic fields from a permanent magnet located within a floatation device. Tanks used in fueling environments are usually located underground. Liquid fuels such as gasoline or diesel are stored in bulk until they are dispensed to customers by means of the station's dispensing equipment. Environmental compliances require that monitoring systems be in place to determine inventory and leakage.
Similar to its use in this invention, a magnetostrictive probe is fitted into a tank, and is comprised of a shaft that protrudes over the height of the entire tank. Detections and logic circuitries are located inside of a canister, usually situated on top of the shaft. The probe and other accessories, like floats, are introduced into the tank via a riser pipe connected to the tank. The probe is then connected to a monitoring system, to which data from the probe is sent, in order to determine the status of the tank. The means of ascertaining the levels, “fuel” and “water”, are commonly accomplished through the utilization of floating bodies, each carrying a magnet. The floats are often constructed of materials such as Nitrophyl, Buna-N, Urethane and Stainless Steel. In the tank, floats are calibrated to have densities that are less than the fuel they are intended to monitor in order to float at the surface of said liquid; in this case gasoline products. Floats are allowed to sink into a fuel layer to stop at the interface of another fluid where the buoyancy forces exerted by the combined liquids, one liquid affecting the upper portion of the float and the other liquid affecting the lower portion, matches the weight of the float in question. In this instance, the float remains at the interface of the two liquids. By this method, systems are not limited to only two floats. A multiplicity of such floats could be adapted into a single probe intended to be used in a tank having various fluids of different densities.
The magnetostrictive probe detection apparatus is set to locate the presence of a magnet along the shaft by means of an interaction between permanent magnetic fields emanating from a magnet and circumferential fields induced by an electric current pulse into the sonic waveguide which is a nickel-alloy based wire. With the float slidably situated along the probe's shaft and carrying a magnet, the system is able to determine the exact position of the float along the shaft. This is accomplished by means of the known propagation velocity of the twist in said wire resulting from the interaction between the two magnetic fields previously mentioned. The delta time from when the current pulse was applied to the ferromagnetic wire to the time a resulting twist is detected by the wire twist sensing pickup apparatus represents the time interval taken for the wave to propagate along the wire medium from its origin. When that delta time is divided by the known propagation velocity of the twist in that particular wire, or its gradient, the magnet's exact location in relation to the detector is then calculated by the system.
  D  =            Δ      ⁢                          ⁢      T        G  Where:    D=Distance being measured    ΔT=Time from when the current pulse was launched in the wire to when the twist is received by the pickup in Seconds    G=Gradient or the wave velocity of propagation in Sec/inch
The detection apparatus could be a pickup coil, a piezoelectric crystal or received by means of mode converter tapes coupled to the waveguide. In the case of the pickup coil, a twist emanating from the two magnetic fields travels the wire as torsional waves and arrives at the coil base and causes disturbances in the previously aligned domains in the said waveguide, which in turn induces a voltage into the coil. In the case of the piezoelectric detector, the arriving mechanical wave causes an oscillation of the crystal. That oscillation produces a similar effect of inducing a voltage in the crystal. The mode converters translate the torsional waves into longitudinal waves that get measured by means of coils or crystals to produce a voltage. In all cases, the resulting signal is amplified and detection circuits are set to process the signal out of which further calculations are made.
In prior art, while the measurement obtained is relative to where the float is located to where the pickup coil is, namely inside the canister at the top of the tank, it does not tell exactly the level in relation to the bottom of the tank. To ascertain this, various techniques are used. In some cases, the distance measurement to a pickup located in the canister is made, and is subtracted from a predetermined tank diameter in order to relate the height from the tank's bottom. Other techniques make use of a reflected termination at the bottom of the wire to ascertain the end of the probe. While this technique offers the benefit of having the distance resolution doubled, the pulses still have to travel up to the canister to be measured as in the first case. Meanwhile, the reflected pulse introduces an error in the absolute measurement. If not accounted for it could result in a much larger error in the determination of the tank's bottom than in the first case. The more distance a signal has to travel, the more attenuation will result. Because of this, reflected termination is not practical to be used for very long probes. In other instances, the use of a reference magnet located at the foot of the probe is made. A gate is formed from the time the signal from the fuel float arrives at the sensing element to when the reference is detected. That time differential approach is a more direct measurement in relation to the bottom of the tank than the previously described methods, but not without some drawbacks.
This reference magnet may be situated internally or externally to the probe shaft. If the reference magnet is not located at the very tip of the wire, which is only possible if it is situated inside the pipe, or the probe is not resting at the bottom of the tank, there may result some drifting in position measurement due to the wire roving about that reference point. When this happens, the measurement is not stable since temperature changes seen by the whole probe will cause the system to shift in various parameters, including probe length, and introduce errors in the measurement. If the reference magnet is located externally to the probe, the obtained height will be affected by temperature drifts.